1. Field of the Invention
The invention relates to a device for generating ions in gas streams for reducing electrostatic charges which, on sensitive products, such as e.g. microchips, films, magnetic plates, laser storage plates and printed circuit boards, in the case of an uncontrolled discharge lead to destruction or increased particle deposition.
2. Background of the Invention
In the manufacture of highly integrated semi-conductor components, with laser and magnetic storage plates and with other products having microstructures in the resolution range of one micrometer and less, both particle contamination and uncontrolled, electrical discharges lead to considerable quality losses. The term microstructures here also covers sensitive plastic films or surfaces in general, in which the deposition of micro particles lead to quality losses. Electro-static charges are the cause of the damage. Such manufacturing processes, for example, take place in clean rooms, whose air is prefiltered to a very high level and flows through the clean room in a low turbulence, piston-like displacement flow. The air flowing into such clean rooms can be filtered to such a high level that virtually no particles pass via, the air flow, into the clean room. The particles produced during manufacture, largely result from the production process itself or are caused by the operating personnel. The device, according to the invention, can also be operated at restrictive work places or stations with specially produced air flow.
The charges are produced by friction, electrostatic induction, or capacitive processes and are unavoidable during the movement of the product, particularly on insulating surfaces. Charge densities can occur, which lead to voltages of several thousand volts. These charged surfaces, by means of electrostatic forces, increasingly attract aerosols, particularly charged aerosols.
In the case of surfaces charged with 500 V, there is approximately a 20X particle deposition compared with a neutral surface. However, such surface charges can be discharged in uncontrolled manner over the microstructures, which can either be destroyed by an electric breakdown or by high current densities. Sensitive metal oxide semiconductor structures on silicon chips can be destroyed by discharges of voltages of around 50 V.
The charging of insulating surfaces on the product and increased particle deposition can be prevented through the air flow containing ions having a positive and negative sign. Thus, charges are compensated both on airborne particles and on the product surfaces. There can be no uncontrolled discharges over the microstructures. Surface discharges are reduced by a controlled discharge over air ions. In the case of electrostatically sensitive products a uniform distribution of positive and negative ions is particularly important.
For generating positive and negative air ions, it is known to use the Townsend gas discharge in the non-uniform electrical field of needle points of wires. A device for generating ions on points is disclosed by U.S. Pat. No. 1,356,211, while DE-OS No. 28 09 054 describes a device for generating ions on wires. In the vicinity of the points or wire surface a discharge zone is formed with an extension of approximately 0.5 mm, in which the gas molecules are ionized. With increasing distance from the discharge zone the speed decreases as a result of the field which is becoming ever weaker. A condition which must be fulfilled for ensuring that the ions can be carried away by the air stream is that their speed is the non-uniform field drops to a value which is lower than the air speed. For igniting a gas discharge on highly curved surfaces a voltage of 6 to 7 kV is necessary. When operating such ionizers with a voltage of approximately 10 kV, the speed of the ions decreases within 50 to 100 cm to a value below 1 m/sec. The standard air flow rate at clean work stations is approximately 0.5 m/sec. It becomes clear from what has been stated hereinbefore that for the distribution of the ions in the air flow, there is a close connection between the air speed on the one hand and the time pattern of the high voltage linked with the charge electro-geometry.
Conventional ionizers operate with voltages between 10 and 20 kV. The time behavior of the voltage is either uniform (FIG. 1c), a signwave voltage (FIG. 1a) of 50 to 60 Hz or a rectangular voltage gradient (FIG. 1c).
It is known that for the same field geometry of the discharge and the same voltage, more ions are generated at the negative emitter than at the positive emitter. As ionizers can only fulfil their surface discharge neutralization function if the same number of positive and negative ions is introduced into the air flow, the sinusoidal a.c. voltage is disadvantageous for the supply of emitters, whereas, in the case of a rectangular voltage gradient and a d.c. voltage supply, it is possible to geneate ions with a compensated polarity balance by setting the corresponding d.c. voltage level.
The rectangular voltage gradient and the sinusoidal a.c. voltage suffer from the disadvantages that the switching of the peak polarity takes place at times which are short compared with the flow rate of the air. In this case ions introduced into the air are returned to the point through the rapid polarity change and are ineffective for air ionization, thus the efficiency of ion emission is also impaired. Efficiency is here understood to mean the ratio of the number of ions entering the air flow to the total number of ions generated at the point.
These disadvantages increase the current loading of the point electrodes. In the case of high current loading of the point electrodes, there is an increased material removal and consequently an incease of the radius at the points, as well as increased accumulation of particles at the point. Thus, ion generation decreases with the reduction of the non-homogeneous field. Therefore time-constant operating conditions are called into question. In practice, these disadvantages are corrected by increasing the operating voltage, which speeds up the described disadvantages.
Increased current loading by return transit is also not prevented in known systems, in that in each case two point groups are separately supplied with d.c. voltage. In this case the potential difference between the points is approximately 20 kV and the spacing between the points must be correspondingly large at approximately 30 cm. Consequently the average ionic velocity remains so large that only a small ion proportion from the marginal zones of the electric field is taken up by the air flow. Therefore, the same disadvantages must be expected as in the case of a.c. voltage-operated ionizers. The construction of planar ionozers, such as can, for example, be fitted in large-areas like those found under the ceiling of clean rooms, leads to a locally discontinuous ion generation. In the boundary region of ionizers supplied in this way there are excesses of one ion polarity which, contrary to the actual function of ionziers, can lead to additional charges. It is even more disadvantageous that the constant field strengths parallel to the electrode plane produced between such electrodes, supplied with d.c. voltage and fitted in the cross-sectional plane of the air flow lead, on the outflow side to the separation of negative and positive ions. Such a separation can lead to charges of several hundred volts due to the excess of ions of one polarity.
Through operational experience with ionizers in clean rooms, for example, of class 10 according to U.S. Federal Standard 209c with particularly high requirements, operational disadvantages have been found in the case of the three operating modes of ionizers described in FIG. 1. These disadvantages relate inter alia to the wearing away of the points, the introduction of metallic point material into the clean room air and to the accumulation of contaminants on the points, as well as electrochemical conversion processes of gaseous products into solid particles. According to the latest research of B. Y. Liu et al, Tex. Instr. Corp: Characterizaton of Electroinc Ionziers for Clean Rooms; IES 1985, in the clean room air there are up to an additional 1.5.times.10.sup.6 particles per m.sup.3. However, in top-quality clean rooms, particle concentrations around 300 particles per m.sup.3 and less are sought.